The objective of the project is to inrease our knowledge of the antigens that are present on the cell surface of Streptococcus mutans since this knowledge is an essential basis for immunoprophylactic studies related to dental caries. Specifically, the rhamnose-glucose polymers (RGP), present in the cell walls of S. mutans serotypes a, d, and g strains, are being studied. These wall antigens have not previously been characterized. To date, RGPs have been chemically and immunologically characterized from S. mutans strains B13 and 6715-T2, serotypes d and g respectively. RGP/B13 and RGP/6715-T2 are immunologically identical, whereas they are both chemically and immunologically distinct from serotype d and g polysaccharide antigens. EGP was recently isolated from serotype a cell walls and is presently being characterized. In studies of sixty antisera produced against chemostat grown serotype c cells, all sixty antisera crossreacted with RGP/B13 polymer. These data and studies using antisera prepared to purified c antigen suggest that crossreacting antibodies to RGP/B13 were related to a site on th c antigen and that variations in culture conditions had little effect on c antigen and its crossreacting determinant. The wide cross-reactivity of RGP among S. mutans serotypes may be useful in the development of a preventative dental caries vaccine.